This invention relates to a fluidization nozzle and a fluidized bed system utilizing same and, more particularly, to such a nozzle and system in which a bed of particulate material is fluidized by the introduction of air into the bed through the nozzle without the particulate material in the bed being allowed to backsift through the nozzle.
Fluidized bed reactors, such as gasifiers, steam generators, combustors, and the like, are well known. In these arrangements, pressurized air or other fluidizing media is passed, via a plurality of nozzles, through a bed of particulate material, including a fossil fuel such as coal and an adsorbent for the sulfur generated as a result of combustion of the coal, to fluidize the bed and to promote the combustion of the fuel at a relatively low temperature. The heat produced by the fluidized bed is utilized in various applications such as the generation of steam, which results in an attractive combination of high heat release, high sulfur adsorption, low nitrogen oxide emissions and fuel flexibility.
The most typical fluidized bed reactor is commonly referred to as a "bubbling" fluidized bed in which the bed of particulate material has a relatively high density and a well-defined, or discrete, upper surface.
Other types of fluidized bed reactors utilize a "circulating" fluidized bed. According to these processes, the fluidized bed density is well below that of a typical bubbling fluidized bed, the air velocity is greater than that of a bubbling bed, and the air entrains a substantial amount of particulate solids and is substantially saturated therewith.
In the event the reactor is in the form of a steam generator, the walls of the reactor are formed by a plurality of heat transfer tubes. The heat produced by combustion within the fluidized bed is transferred to a heat exchange medium, such as water, circulating through the tubes. The heat transfer tubes are usually connected to a natural water circulation circuitry, including a steam drum, which separates the water from the converted steam, which is routed either to a steam turbine or the like.
In both the bubbling and circulating fluidized bed arrangements, an air plenum is disposed below an air distributor plate, or grid, for supplying pressurized air to the bed. In some arrangements a plurality of air nozzles are supported by the plate and extend above the plate into the bed. The nozzles receive air from the plenum and discharge the air into the bed in a predetermined flow pattern.
However, the solids from the bed can backflow or backsift through the nozzles into the air plenum, especially in connection with circulating fluidized beds when the unit is suddenly shut down while operating at full loads. As a result, the solids will accumulate in the air plenum and block air flow through the nozzles.